sounds like you are twisting your wrist slightly outwards...take a few practice swings first and apply the following:

get ready to swing by pressing the A button but keep your hand perfectly still and watch the club for signs of movement. if there is movement then let go of A and try again. If there is no movement at all in the club and you havent moved your wrist or hand then take your normal swing (try not to swing across your body). If your bring your hand across your body i.e. you swing in an arc then the wiimote will sense that you have rotated your wrists and you will hook, slice, draw or fade. swing back in a straight line and swing forward in a straight line. dont try and put too much force intoyour swing neither i.e. dont swing too fast. take a steady slow swing. the momentum will give you anywhere between 95% and 105%